SCIENCE
Shadows in the afterglow of the Big Bang reveal invisible cosmic structures Story-level
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north400,000 years after the Big Bang, the primordial plasma of the infant universe cooled enough for the first atoms to fuse, leaving space for the embedded radiation to rise freely. That light, the cosmic microwave background (CMB), continues to stream across the sky in all directions, broadcasting a snapshot of the early universe that is captured by dedicated telescopes and even revealed in static on old cathode ray televisions.
After scientists discovered CMB radiation in 1965, they meticulously mapped its small temperature variations, which showed the exact state of the cosmos when it was a mere foamy plasma. Now they are reusing the CMB data to catalog the large-scale structures that developed over billions of years as the universe matured.
“That light experienced much of the history of the universe, and by seeing how it has changed, we can learn about different epochs,” he said. kimmy wucosmologist at SLAC’s National Accelerator Laboratory.
Over the course of its nearly 14 billion-year journey, light from the CMB has been stretched, compressed, and deformed by all the matter that got in its way. Cosmologists are beginning to look beyond the primary fluctuations in CMB light to the secondary traces left by interactions with galaxies and other cosmic structures. From these signals, they are getting a clearer view of the distribution of both ordinary matter, everything that is made up of atomic parts, and the mysterious dark matter. In turn, those insights are helping to solve some longstanding cosmological mysteries and raising some new ones.
“We are realizing that the CMB doesn’t just tell us about the initial conditions of the universe. It also tells us about the galaxies themselves,” he said. emmanuel schaan, also a cosmologist at SLAC. “And that turns out to be really powerful.”
a universe of shadows
Standard optical surveys, which track the light emitted by stars, miss most of the underlying mass of galaxies. This is because the vast majority of the total matter content of the universe is invisible to telescopes, hiding out of sight either as clumps of dark matter or as the diffuse ionized gas that binds galaxies together. But both the dark matter and the scattered gas leave detectable traces in the magnification and color of the incoming CMB light.
“The universe is really a shadow theater in which galaxies are the stars and the CMB is the background light,” Schaan said.
Many of the shadow players are now taking over.
When particles of light, or photons, from the CMB scatter electrons in the gas between the galaxies, they rise to higher energies. Furthermore, if those galaxies are in motion with respect to the expanding universe, the CMB photons get a second shift in energy, either up or down, depending on the relative motion of the cluster.
This pair of effects, known respectively as Sunyaev-Zel’dovich (SZ) thermal and kinematic effects, were first theorized in the late 1960s and have been more accurately detected in the past decade. Together, the SZ effects leave a characteristic signature that can be extracted from CMB images, allowing scientists to map the location and temperature of all ordinary matter in the universe.
Finally, a third effect known as weak gravitational lensing warps the path of CMB light when it travels near massive objects, distorting the CMB as if viewed through the bottom of a wine glass. Unlike SZ effects, the lens is sensitive to all matter, dark or not.
Taken together, these effects allow cosmologists to separate ordinary matter from dark matter. Scientists can then overlay these maps with images from galaxy surveys to measure cosmic distances and even star formation trail.
In buddy documents in 2021, a team led by Schaan and Stephanie Amodeo, which is now at the Strasbourg Astronomical Observatory in France, put this approach to work. They examined CMB data taken by the European Space Agency Planck satellite and the land based Atacama Cosmology Telescope, then stacked on top of those maps a further optical survey of nearly 500,000 galaxies. The technique allowed them to measure the alignment of ordinary matter and dark matter.
The analysis showed that the region’s gas was not hugging its supporting dark matter web as tightly as many models predicted. Instead, it suggests that supernova explosions and the buildup of supermassive black holes forced the gas away from its dark matter nodes, spreading it out in a way that was too thin and cold for conventional telescopes to detect.
Detecting that diffuse gas in the CMB’s shadows has helped scientists further address the so-called missing baryon problem. It has also provided estimates of the strength and temperature of the scattering explosions, data that scientists are now using to refine their models of galaxy evolution and the large-scale structure of the universe.
In recent years, cosmologists have been puzzled by the fact that the observed distribution of matter in the modern universe is smoother than theory predicts. If the explosions that recycle intergalactic gas are more energetic than scientists assumed, such as recent work by Schaan, Amodeo, and others seems to suggest that these explosions could be partly responsible for spreading matter more evenly throughout the universe, he said. colin hill, a Columbia University cosmologist who also works on CMB signatures. In the coming months, Hill and his colleagues at the Atacama Cosmology Telescope plan to reveal an updated CMB shadow map with a noticeable jump in both sky coverage and sensitivity.
“We’ve only just begun to scratch the surface of what you can do with this map,” Hill said. “It’s a sensational improvement over anything that came before it. It’s hard to believe it’s real.”
shadows of the unknown
The CMB was a key piece of evidence that helped establish the standard model of cosmology, the central framework that researchers use to understand the origin, composition, and shape of the universe. But CMB’s backlight studies are now threatening to poke holes in that story.
“This paradigm really survived the test of precision measurements, until recently,” he said. Eiichiro Komatsua cosmologist at the Max Planck Institute for Astrophysics who worked to establish the theory as a member of the Wilkinson Microwave Anisotropy Probe, which mapped the CMB between 2001 and 2010. “We may be at the crossroads…of a new model of the universe.”
For the past two years, Komatsu and his colleagues have been investigating hints of a new character on the shadow theater stage. The signal appears in the polarization, or orientation, of CMB light waves, which according to the standard model of cosmology should remain constant as the waves travel through the universe. But how theorized three decades ago by Sean Carroll and his colleagues, that polarization could be rotated by a field of dark matter, dark energy, or some entirely new particle. Such a field would cause photons of different polarizations to travel at different speeds and rotate the net polarization of light, a property known as “birefringence” that is shared by certain crystals, such as those that power LCD screens. In 2020, the Komatsu team reported finding a small rotation in the polarization of the CMB, about 0.35 degrees. A follow-up study published last year reinforced that previous result.
If the polarization study or another result related to the distribution of galaxies is confirmed, it would imply that the universe does not look the same in all directions for all observers. For Hill and many others, both results are tantalizing but not yet definitive. Follow-up studies are underway to investigate these clues and rule out possible confounding effects. Some have even proposed a dedicated spaceship “backlit astronomy” that would further inspect the various shades.
“Five to 10 years ago, people thought the cosmology was done,” Komatsu said. “That is changing now. We are entering a new era.”
This article was originally posted about him quantum abstractions Blog.
Lead image: “The universe is really a shadow theater in which galaxies are the stars, and the CMB is the backlight,” said cosmologist Emmanuel Schaan. Credit: Kristina Armitage / Quanta Magazine.
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